Echo suppressor



Dec. 17, 1929. H. NYQulsT ET AL. 1,739,606

ECHO SUPPRESSOR original Filed oct. 11, 1927 I T E STT HARRY' NYQUIST, OF MILBURN, ANI) KENNETH W. PI-LEGER, OF ARLINGTON, NEW JERSEY, ASSIGNORS TO AMERICAN TELEPHONE AND TELEGRAPH 4COMIEAlNY, A

CORPORATION OF NEW YORK sono' surrmisetm` y l Application filed October 11, 1927, Serial No. 225,571. Renewed August 31, 1928.

pathvadapted for transmission in the opposite direction, and these two paths are connected' at each end of the' circuit, 1n the case of i all-wire systems, through a hybrid coil to a two-wire line, which .hneis approximately balanced by asuitable network. In some sys--` tems the transmission is in part overa radio link, and in such cases theyfour-wire section maybe connected at one end to a two-wire line through a hybrid coil and at the other end to radio apparatus for transmission overand reception from the radio link.

Since the balance of the two-wire line and the associated network is not perfect, there is the likelihood that currents flowing in one of4 the transmission paths of the four-Wire circuit will be reiiected back from the two-Wire line over the other transmission path, producing echoes. In order to suppress these echoes voice-operated means have been devised which respond to voice currents in one path and function to short-circuit or otherwise disable the opposite path. For a, full disclosure of echo suppressor arrangements as applied to all-wirecircuits, reference mig be had to an article by A. B. Clark and R.

Mathes in the Journal of the American Institute of Electrical Engineers, June, 1925, pages 618 to 626.

- If-the transmissionlines of the telephone system run arallel to power lines or are subjected to ot er interference, the current reg sponsive devices for disabling one path of the four-wire circuit may be falsely operated by the interfering currents in the voice channels. The object of the applicants invention is to provide a voice-operated echo suppressing arrangement which is free `from operation by one or more frequencies and their harmonics, but will respond readily to all other frequen cies within the range contemplated.

knownV amplifier detector-device for controlling aiswitch to short-circuit one transmission path, and use a speaking tube and a suitable arrangement of telephone receivers and transmitters. so positioned with respect to the speaking tube that the waves correspondingto thel frequenc to be barred and to the harmonies of that re uency will be cancelled out and will have no e ect on the amplifier-detectext', as will be more fully disclosed hereina er.

In .the applicants companion application filed as of even date herewith, there are disclosedl the broad idea of balancing out certain frequencies in the input to the amplifier detector. and a specific arrangement for practicing 'the idea, this arrangement involving the use oa telegraphone or an vequivalent device. In the present application there is disclosed a di'erent and simplearrangement'for accomplishing the same object to which the companion application is directed.

The novelfeatures and the advantages of the piesentinvention will he more clearly understood? when the following description of one embodiment thereof is read with reference to the accompanying drawing.

Figure 1 of the drawing shows diagrammatically and in association with two lines adapted for transmission in opposite directions?l desirable speaking tub arrangement for protecting the voice-opera ed apparatus against operation by one or mor frequencies and their harmonics.

Fig. t 2 of the drawing shows by wave curves howin the speaking tube of Fig. 1 a single waveQand one of its frequencies are cancelled out before they can aifect the input to theamplifier detector.

3 and 4 show alternative arrangements of the telephone receivers and transmitters with respect to the speaking tube of Fig. 1,. 9

ik'e characters of reference in the several figures of the drawing designate correspond- V1n elements,

50 In general, the applicants employ the well ing part of a two-way telephone system, as indicated and briey discussed above. Line L1V is adapted for-transmission from station W at the left to station E at the right, and

. phone M at they r were opposite direction.

line L2 is adapted for transmission in the' It isV understood, `of course, that these voice paths are associated at one end directly and at the other end either diectly or over the radio link to a twowire line through the usual hybrid coil, as clearly disclosed ,in the above identified/'artic e.

The voice-operated device shown comprises a vacuum tube amplifier A, a vacuum tube detector D associated therewith, and a relayv S controlled by the operation of the detector for short-circuiting the line L2 in response to voice currents in the line L1. It is to be understood, of course, that for the suppreslsion of echoes in the opposite direction similar apparatus is provided to short circuit the line L1 in response to voice currents Howing in L2. If, now, interfering currents, such as currents from power lines having a certain frequency, dow in the line L1, they may operate the amplifier detector A-D and the associated switching device to produce a false short-'circuiting of the line L2.

ln order to prevent the operation of the echo suppressor by currents of one or more given frequencies or their harmonics, the applicants arranged a speaking tube, the 'receivers its and R2 and a microphone as shown in Fig. 1. The receivers R1 and R2 are positioned in the side-wall of the speaking tube and are connected across the line L, and... in series one withthe other so as to give. soundy waves 180 degrees out of phase. The recei are separated by a distance L, which is determined as described hereinafter. 1n response to current lowin the line L1 sound Waves are produced in the speaking tube through the receivers and Vare transmitted throughthe air Within the tube to the microight end of the tupe. The waves, ofcourse, rill also travel to the left, but they are absorbed at the .left endvof' the tube Without redectien by means of a device which acts like a tube of ininite length, this device consisting of a iong cui?. small tub-e. The main portion ef the tuhe aGuild be sufficiently large in ianieter thatthere will be negligibie attenuation and the yelocity ali `'fn-cgiencies wilt he approximately the saine.

The distance f2,

' i" det-ern ined in accert tace with the ngth er f te the frequency protestiert is d such that nce L undamentai ie harmonics. i eviti rea "e ieaeoe more fundamental Vinterfering frequencies. Y

When these conditions are met, the interferingwaves will be cancelled out at the right end of the speaking tube and will not affect the microphone h1. i f

The case of protection against two inter'- fering frequencies and their harmonics being considered, for instance, the distance L must equal Aim, which is equal to Agn, where )t1 and )t2 are the Wave lengths of the fundamental interfering frequencies and m and n are whole numbers. The numbers m and ln. should be as small as possible for tworeasons: the load on the power wires may cause a the microphone receives Waves from R1 and R2- which are unequal 1n magnitude; 1n the -second place,.tlie length of tubing should be reduced to the minimum. y

Even when the length of the tube is reduced tothe minimum, this length may be necessarily rather great. -Letit be assumed, for in# stance, that protection is desired against interference from two power lines, one carrying (iO-cycle current and the other 25-cycle current. A (S0-cycle sound Wave is approximately 18 feet-in length,'and a 25-cycle sound :rave is approximately 43.2 feet in length. At their minimum values m must equal 12 and n must equal 5, and, accordingly, a tube if the reduction of the length of tubing is not an important consideration, the tube length indicated above may be multiplied by any Whole number.

The cancellation of the waves corresponding to the interfering frequencies andtheir harmonics will be more clearly understood' from an examination of Fig, 2 of the drawing. in this figure the distance between R1 and ltzf equals 29. and the unbroken. curves represent the fundamental waves sent through the tube 'to the microphone M from the receivers of and R2. Due to the spacing of the two receivers these waves are 180 degrees out of ph and cancel each other at the microphone end the tube.. Likewise, the second harmonic the frequency represented by the unbn ten curve is resented, by the-'breiten curse seen. thatinis har manic, like fui te tal, is canceiied out the microphone eno. the t te.

is, cous-e, 'niet n' that the ar- In the first place, sudden changes ini short interval of time to elapse during which '85 ment of telephone receivers and transmitters,

or the equivalents, so connected and so positioned with respect to the tube that the waves corresponding to the frequency or frequencies against Whichprotection is desired and to the harmonics of that frequency or those fre-v quencie's will be cancelled out of the input to the amplifier-detector `or other controlling means.

The receivers R1 andRg, if two receivers are employed, are not necessarily connected in seriegvwith` one another, but any connection giving the phase separation of 180 for the cltlisen frequency or frequencies Will be suita e. Y

Figs. 3 and4 of thedrawing show arrangements, for example, which diti'er from that shown in Figs. 1 and 2, but will give the desired result, assuming, of course, that the requirement stated above as to the distance L is observed.

In Fig. 3, the receivers R1 and R2 are mounted wit-hin the tube,\and the microphone M is positioned in the side wall of the tube.

In Fig. 4, a single receiver is mounted at one end of the speaking tube, and two microphones are mounted in the side wall of the tube and are spaced with reference to the wave length or wave lengths-corresponding tothe frequency or frequencies to be barred` and are connected in series opposing relation to give the desired cancellation in the input to l the amplifier-detector.

While the invention has been disclosed in certain specific arrangements which are deemed desirable, it is to be understood that it may be embodied in other and widely different forms Within the truescope of the invention as defined in the appended claims.

What is claimed is:

1. In association with two telephone lines adapted for transmission in opposite directions, controlling means responsive to currents in one of said lines, switching means controlled thereby for disabling the opposite line, a speaking tube, means for converting the electric waves in the said one of the telephone lines into sound waves in said speaking tube, and means for converting the sound waves in said speaking tube into e ectric waves in the input of said controlling means, said convertingmeans being so arranged and positioned with respect to said speaking tube that Waves of one or more given frequencies and the harmonics thereof are cancelled and have no effect on said controlling means.

2. In association with two telephone lines adapted for transmission in opposite d1rections, controlling means responsive to currents in one of said lines, switching means controlled thereby for disabling the opposite verting means being so arranged and positioned with respect to said speaking tube that Waves o'f one or more given frequencies and the harmonics thereof are cancelled and have no ei'ect on said controlling means, and means at one end of said speaking tube for absorbing the sound waves therein without reflection.

3. In association with two telephone lines adapted for transmission in opposite directions, controlling means responsive to currents in one of said lines, switching means controlled thereby for disabling the opposite line, a speaking tube, means for converting the electric waves in the said one of the telephone lines into sound waves in said speaking tube, means for converting the sound waves in said speaking tube into electric Waves in the input of said controlling means, and means for cancellingout in said speaking tube vthe waves of one or more given frequencies and the harmonics thereof. Y

4. In association with two telephone lines adapted for transmission in opposite directions, controlling means responsive to currents in one of said 'lines,`switching means controlled thereby for disabling the opposite line, a speaking tube, means for converting the electric waves in the said one ofthe telephone lines into sound waves in said speaking tube,v means for converting the sound waves in said speaking tube into electric waves in the input of said controlling means, means at one end of said speaking tube for absorbing the sound Waves therein without reflection, and means for cancelling out in said speaking tube the waves of one or more given frequencies and the harmonics thereof.

5. In association with two telephone lines adapted for transmission i opposite directions, controlling means responsive to currents i'n one of said lines, switching means controlled thereby for disabling the opposite i line, a sound propagating medium, means for converting the electric waves in the said one of the telephone lines into sound waves in said sound propagating medium, and means for converting the sound waves`in said sound propagatingI medium into electric Waves in the vinput of said controlling means, said converting means being so arranged 'and positioned with respect to said sound propagating medium that waves of one or more viven frequencies and the harmonics thereof are substantially cancelled and have no effect on said controlling means.

6. In association with two telephone lines adapted for transmission in opposite directions, controlling means responsive to cur- 4 i megane rents in one of said lines, switching means controlled thereby for disabling the opposite line, a soundv propagating medium, means for converting the electric Waves in the said one 5 of the telephone lines into sound WavesV in said lsound propagatingmedium, means for converting sound Waves in said sound propagating medium into electric Waves in the in- 1 put of said controlling means, said converting means being so arranged and positioned with respect to said sound propagating medium that Waves ofl one or more given frequencies and the harmonics thereof are substantially cancelled and have no effect on said controlling means, and means at one end of said sound propagating medium for absorbing the sound Waves therein Without reflection.

7. In association with two telephone lines adapted for transmission in opposite directions, controlling means responsive to currents in one of said lines, switching means controlled thereby for disabling the opposite line, a sound propagating medium, means for converting electric Waves in the said one of the telephone lines into sound Waves in said sound propagating medium, means forpconverting sound Waves in said sound propa. gating medium into electric Waves in -the input o f said controlling means, and means as sociated with said sound propagating medium for substantially' cancelling the waves ofone or more given frequencies and the harmonics thereof. l 8. In association with two telephone lines adapted for transmission in opposite directions, controlling means responsive to currents in one of said lines, switching means controlled thereby for disabling the'opposite line, a sound propagating medium, means '4o for converting the electric Waves in the said one of the telephone lines into sound Waves in said sound propagating medium, meansI for converting the sound Waves in said sound propagating medium into electric Waves in the input of said controlling means, means at one end of said sound propagating medium for absorbing the sound Waves therein Without reflection, and-means associated with said sound propagating medium for substantially cancelling the Waves of one or more given frequencies and the harmonics thereof.

In testimony whereof, We have signed our names to this specification this 8th' day of October, 1927. s vHARRY NYQUIST.

KENNETH A W. PFLEGER. 

